Malignant epithelial tumors are the main cancer-related cause of human death. These solid tumors frequently exhibit significant stromal reactions such as the so-called “desmoplastic stroma” or “reactive stroma”, which represents 20-60% of total tumor mass and is characterized by the existence of large numbers of stromal cells and dense extracellular matrix (ECM). Recent studies have indicated the tumor-promoting roles of stromal cells, as exemplified by vascular cells, immune cells, fibroblasts, myofibroblasts, adipocytes and bone marrow-derived progenitors (1-6). In particular, considerable numbers of cancer-associated fibroblasts (CAFs) are frequently observed within tumor-associated stroma of various human cancers, including breast, lung, colon, and pancreas carcinomas (14,15). Interacting coordinately with the different components of the stroma, CAFs have the ability to promote neoangiogenesis and tumor growth; CAFs have also been shown as crucial for the development of aggressive tumors and tumor invasiveness during cancer progression (16-25); CAFs facilitate the spreading and infiltration of tumor cells in distant organs, thus contributing to formation of metastases. Importantly, the relevance of stromal cells to the failure of systemic drug delivery to tumors and to the development of drug resistance has also been indicated (7-11).
The identification of cellular and molecular targets abrogating stromal-tumor cell interactions and thus attenuating tumorigenesis is currently one of the most important subjects in translational oncology. Indeed, targeting the peritumoral stroma is a fairly new strategy to treat metastatic tumors, which represent more than 90% of cancer patient mortality: only a few products have obtained therapeutic approval up to now, most of them being anti-angiogenic drugs (Avastin®; 26). Identifying and targeting other new molecules within the tumor microenvironment is then essential for increasing the efficacy of conventional therapies in combination with the stroma-based therapeutic approaches, and represent a powerful approach for cancer and metastasis treatment (12, 13).
Monoclonal antibody (MAb)-based drugs represent a great promise in the fight against cancer. This is because they allow the treatment to be aimed at a molecular level in a precise and specific way. These advantages, together with their commercial appeal (short development times, restricted competence and being easily exportable to other cancer types once they have been approved), have pushed many pharmaceutical companies to invest heavily in the development of new antibody-based molecules, as well as in the in-licensing of new molecules or technologies from biotech companies.
However, despite the clinical success of therapeutic antibodies, naked MAbs targeting cell surface tumor antigens rarely present sufficient efficacy on their own. To increase the low activity of the MAbs, novel strategies are focusing on binding them to toxic molecules. Plant and bacterial toxins as well as small chemotherapeutic molecules can be good candidates, since they are very potent and active in very small quantities.
The field of immunotoxins (ITs) and Antibody-Drug conjugates (ADCs) for the treatment of cancer has recently experienced a growing development activity by pharmaceutical companies, due to the technological advances performed during the last years, aimed at solving the problems they initially presented about immunogenicity, undesirable toxicity, production, half-life and resistance.
Immunoconjugates are made of a human, humanized or chimeric recombinant antibody, covalently linked to a cytotoxic drug. The main goal of such a structure is joining the power of small cytotoxic (300 to 1000 Da) and the high specificity of tumor-associated antigen targeted (TAA) MAbs.
The Ab must be very selective to reach the antigen, whose expression must be restricted in normal cells. The Ab also must be internalized efficiently into the cancerous cells.
The cytotoxic agent selected as the effector moiety must kill cells only after internalization and release into the cell cytoplasm. The most commonly used payloads in ADCs are DNA-harming drugs such as calicheamicins, duocarmicins, or microtubule-targeting compounds like auristatins and maitansinoids.
The Ab-cytotoxic linkers are designed to be stable systemically and to release the cytotoxic within the target cells.
TAAs are frequently cell membrane proteins that are overexpressed in diseased tissues or at least expressed sufficiently to facilitate the internalization-activated cytotoxicity. Ideally the antigen presents a restricted expression in normal tissues with a low or absent expression in vital organs. On top of this, the tumor antigen must be recognized selectively and with high affinity by an Ab.
In many types of human cancer, fibroblast response is characterized by the induction of a cell surface protein, Fibroblast Activating Protein α (FAPα), a serine protease of 95 kDa whose expression is highly restricted to developing organs, wound-healing and tissue remodeling.
FAP presents the following characteristics:                Type II membrane glycoprotein with SER-protease activity (collagenase+DPP)        89% human-murine protein homology        Tumor stroma-expressed in >90% carcinomas (breast, pancreas, lung, bladder and colon)        Transitory and highly restricted expression in normal adult tissues during wound-healing and developing organs.        FAP(+) fibroblasts located closed to tumor vasculature        Very focal expression        Internalization        Implication in extracellular matrix remodeling, tumor growth and metastasis.        
FAP expression has been recently found in Pancreas tumor cells as well as tumor-associated stromal fibroblasts. FAP expression was correlated with shorter patient survival and worse prognosis, suggesting a possible FAP-based autocrine/paracrine loop in this type of tumor (32).
During the last 10 years, Kontermann and Pfizenmaier (IZI, University of Stuttgart, Germany) have developed anti-FAP MAb derivatives against both human and murine proteins (27, 28). They have shown in vitro that anti-FAP scFv immunoliposomes bind specifically FAP+ cells and get internalized (29). In a recent study they demonstrated the anti-tumoral effect of nanoparticles covered with lipids and anti-FAP scFvs and loaded with TNFα (30).
Treatment with murine MAb FAP5-DM1 immunotoxin induced long lasting inhibition of tumor growth and full regression in pancreas and lung cancer xenograft models, without any intolerance-related effect (31).
Despite these advances, there remains an unmet need for further therapeutic strategies for the treatment of tumors, including epithelial tumors, and for components for use in such therapeutic strategies. The present invention addresses these and other needs.